In PCT patent application WO 92/16794 to Witteveen, a combustor has been described which is provided with a device for mixing gaseous fuel with air. The device is provided with a plurality of air passages located in an axially symmetrical housing. Each of the plurality of passages is shaped to discharge air with a vector component in the direction of an axis and a vector component in a tangential direction with respect to the axis. The passages are located around the axis and exit into a rotation chamber having an outer wall. The wall is symmetrical with respect to the axis, has a tapered down portion at the exit ends of the passages and, after the tapered down portion, widens abruptly.
Such a combustor generates very little NOx. With large combustors, especially with a heat generating capacity of more than 1 MW, a problem arises in that, with air flows and/or gaseous fuel flows of relatively large cross-sections in a predetermined length of the mixing traject, the homogeneity of the mixture is somewhat incomplete and NOx generation is somewhat greater. A theoretrical solution would be to have a large number of relatively small flows and/or to have long and complicated mixing trajects with curves of the flows and possibly repeated combinations and splittings of the flows. This solution is, however, expensive and increases the pressure drop in the mixing device.
In the aforementioned PCT application, the phenomenon of vortex break down has been described, which gives a very good mixing and low NOx values by providing that the tapering down portion reduces the diameter to 0.9 to 0.7 of the diameter before tapering down, the widening after the tapering down portion being an increase of the diameter of at least a coefficient of 2.5.
The obtention of vortex break down is secured by features based on a mathematical analysis (see WO 92/16794), which in practice comes down to the preferred region of the angle included between the flow direction and the axis of 50.degree.-70.degree..
PCT patent application WO 93/10397 to Cummings et al shows a combustor with a nozzle which has an end wall and circumferential exits of an air and fuel gas mixture. The mixture is formed in straight pipes having an obliquely tangentially and outwardly directed air flow with one or two fuel gas feed pipes exiting in the wall of the said pipes. The mixture formed in the said pipes is immediately fed to a cylindrical combustion chamber. According to another embodiment of this PCT application, air is fed in from the outside toward the axis of a cylindrical combustion chamber, fuel being fed into the air flow toward the combustion chamber. Both embodiments are more complicated and give a less complete admixture than the invention.